Japanese Patent No. 2740757, for example, is known as a conventional hydraulic control device for a working vehicle and has such a configuration as shown in FIG. 6. A prime mover 1 drives a transmission 3 for making a vehicle travel via a torque converter 2, and drives a first pump 4 and a second pump 5. A discharge opening of the first pump 4 is connected to a steering cylinder 8 via a steering circuit 6, a steering priority valve 7, and a steering changeover valve 8a. A discharge opening of the second pump 5 is connected to a working machine changeover valve 10 via a working machine circuit 9, 9a. An assistance circuit 13 into which part of discharge oil from the first pump 4 flows from the steering priority valve 7 is connected to the working machine circuit 9, 9a. The working machine changeover valve 10 is connected to working machine actuators such as a boom cylinder 11, a bucket cylinder 12, and the like. An unloading valve 14 for unloading discharge oil from the second pump 5 is connected to the working machine circuit 9. A check valve 17 for checking back-flow toward the unloading valve 14 is also connected to the working machine circuit 9 between a portion connected to the assistance circuit 13 and a portion connected to the unloading valve 14. When inputting an actuating signal from a shift-down switch 15 while the transmission 3 is in a second speed gear, a controller 16 outputs a signal for shifting down the transmission 3 to a first speed gear to the transmission 3 and outputs a signal for unloading the second pump 5 to the unloading valve 14.
According to the above configuration, when the shift-down switch 15 is operated during low speed work such as excavating work and while the transmission 3 is in the second speed gear, after inputting an actuating signal from the shift-down switch 15, the controller 16 shifts down the transmission 3 to the first speed gear and unloads the second pump 5 by means of the unloading valve 14. In this situation, only pressure oil discharged from the first pump 4 (the arrow painted over with black in FIG. 6) is supplied to the working machine changeover valve 10 via the steering circuit 6, the steering priority valve 7, the assistance circuit 13, and the working machine circuit 9a. At this time, pressure oil in the assistance circuit 13 and the working machine circuit 9a never flows into the unloading valve 14 owing to the check valve 17. Horse power of the prime mover 1 which becomes unnecessary by unloading the second pump 5 as described above is used for tractive horse power of the working vehicle, thereby increasing digging force of a bucket into earth. As a result, the quantity of earth scooped into the bucket is increased and working ability is enhanced.
According to the aforesaid prior art, tractive force can be increased by the force corresponding to motive power which becomes unnecessary by unloading the second pump 5. However, pressure of the first pump 4 remains unchanged, whereby sufficient working machine lift force such as bucket tilt force or the like can not be obtained. In this case, when earth to be excavated is soft, earth is easy to break even if working machine lift force Fv1 is small as shown in FIG. 7A. Therefore, if tractive force Fh is large, the quantity of earth scooped into the bucket increases, which is effective. However, when earth to be excavated is hard, the digging force of the bucket into the earth to be excavated can not be sufficiently obtained even if the tractive force Fh is increased as shown in FIG. 7B. Hence, the quantity of earth scooped into the bucket does not increase, thus lowering working efficiency. Moreover, a tire slips as shown by the arrow S, thereby causing a disadvantage that abrasion loss is increased, thus shortening the life of the tire. Accordingly, when earth to be excavated is hard, it is requested to increase the tractive force Fh and also increase the working machine lift force like Fv2. For this purpose, it is required to increase the quantity of earth scooped into the bucket while the earth is broken by repeatedly performing working machine operation such that bucket tilt force is put forth by rotating a bucket edge upward and that bucket lift force is put forth by lifting a boom to raise the bucket edge.